Most of the imaging examination methods used in medical technology have been based on X-ray recordings for many years now. In recent years digital recording technologies have increasingly become established in place of conventional radiography based on photographic film. These technologies have the significant advantage that time-consuming film development is no longer required. Images tend instead to be produced by means of electronic image processing. The image is therefore available directly after recording. Digital X-ray recording technologies also offer the advantage of better image quality, possibilities for post-processing the images electronically and the option of dynamic examination, i.e. the recording of moving X-ray images.
The digital X-ray recording technologies used include so-called image-intensifier camera systems, based on television or CCD cameras, storage film systems with integrated or external readout units, systems with a converter film optically linked to CCD cameras or CMOS chips, selenium-based detectors with electrostatic readout systems and solid-state detectors with active readout arrays with direct or indirect X-ray radiation conversion.
Solid-state detectors in particular have been under development for digital X-ray imaging for several years now. Such a detector is based on an active readout array, e.g. of amorphous silicon (a-Si), behind an X-ray converter layer or scintillator layer, e.g. of cesium iodide (CsI). The incident X-ray radiation is first converted to visible light in the scintillator layer. The readout array is divided into a plurality of sensor surfaces in the form of photodiodes which in turn convert said light to electric charge and store it with local resolution. In the case of a so-called direction-conversion solid-state detector an active readout array of active silicon is also used. However this is arranged behind a converter layer, e.g. of selenium, in which the incident X-ray radiation is converted directly to electric charge. This charge is then in turn stored in a sensor surface of the readout array. For the technical background to a solid-state detector, also referred to as a surface image detector, see also M. Spahn et al., “Flachbilddetektoren in der Röntgendiagnostik” (Surface image detectors in X-ray diagnostics), Der Radiologe 43 (2003), pages 340 to 350.